Forthcoming Seminars at F-1

Seminars Archive

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11 Dec 2012
15:00
Martin NussStrongly correlated quantum systems out of equilibrium: A variational cluster approach
Tea room F1. Abstract:

The theoretical understanding of the non-equilibrium behavior of strongly correlated quantum many body systems is a long standing challenge, which has become increasingly relevant with the progress made in the fields of molecular- and nano- electronics, spintronics, spectroscopy or quantum optics and simulation. Besides seeding the fundamental concepts for promising future applications, developing a sound understanding of the plethora of previously unrecognized effects, arising in a non-equilibrium situation, is currently at the forefront of theoretical research.
We report on the development of non-equilibrium cluster perturbation theory [1], and its variational improvement, the non-equilibrium variational cluster approach [2]. The non-equilibrium extensions of the well-established equilibrium theories are based on the Keldysh Greens function method which allows accessing single particle dynamic quantities. These flexible and versatile techniques can in principle be applied to any bosonic and/or fermionic lattice Hamiltonian, including multi-band and multi-impurity systems. Within this framework it is possible to work in the thermodynamic limit and therefore exchange particles with a bath and/or dissipate energy. We will highlight the importance of the self-consistent feedback, introduced in the non-equilibrium variational cluster approach.
We will discuss the performance, open issues and limitations as well as the advantages of the non-equilibrium variational cluster approach on the basis of a single impurity system. Based on the good performance of the variational cluster approach for the single Impurity Anderson model in the equilibrium situation [3], results obtained for the steady state current density as well as the non-equilibrium density of states of a strongly correlated single quantum dot will be presented [4]. These will be benchmarked against data for the quasi stationary state from a real time evolution using matrix product states.

[1] M. Balzer and M. Potthoff, Phys. Rev. B 83, 195132 (2011)
[2] M. Knap, W. von der Linden and E. Arrigoni, Phys. Rev. B 84, 115145 (2011)
[3] M. Nuss, E. Arrigoni, M. Aichhorn and W. von der Linden, Phys. Rev. B 85, 235107 (2012)
[4] M. Nuss, Christoph Heil, Martin Ganahl, Michael Knap, Hans Gerd Evertz, Enrico Arrigoni, Wolfgang von der Linden, to appear in: Phys. Rev. B (2012)

5 Oct 2012
12:00
Markus AichhornCorrelation effects in 3d, 4d, and 5d transition metal oxides
Physics seminar room (106). Abstract:
Strong electron correlations appear when the kinetic energy in a system is comparable or smaller than the repulsive Coulomb interaction energies. Until a few years ago this behavior was attribute in transition metal compounds almost exclusively to materials with open 3d shells, and just a few studies on a limited amount of systems have been done outside of this play ground. Only recently, it has been noted that also systems with 4d or even 5d electronic states in the conduction bands can show very interesting and unexpected behavior. Applying fully ab-initio electronic structure calculations by combining density-functional theory with the dynamical mean field theory, we will discuss the following topics. First, we will focus on ruthenate materials, were heavy electron masses and very low coherence scales have been found in experiments. Second, we will look at the extraordinarily high magnetic transition temperature in technetium compounds. Last but not least, we will discuss the insulating states in iridium compounds. The origin of these effects will be traced back to subtle interplay of several energy scales, i.e. Coulomb, Hund, Spin-Orbit, etc. interactions.

4 Sep 2012
15:00
Artem BadasyanOsmotic Pressure Induced Coupling between Cooperativity and Stability of a Helix-Coil Transition
Physics seminar room (106).

28 Aug 2012
15:00
Thomas PruschkeThe Kondo lattice model in the dynamical mean-field approach
Tea room F1.

20 Jul 2012
11:00
prof. Takami TohyamaResonant inelastic x-ray scattering (RIXS) in cuprates and iron pnictides
Tea room F1.

27 Jun 2012
11:30
Vikram SoniNew Physics from Magnetars
We shall walk you through a plausible theory of the origin of magnetars – neutron stars with surface magnetic fields of 10^14(15) gauss and flares that emit 10^47 ergs of energy in a fraction of a second. In our picture magnetars are most massive neutron stars that carry a magnetized core created by a high density strong interaction phase transition. The magnetic core so created is first screened by the surrounding high conductivity plasma. It is then transported to the crust and breaks through the crust to get out to the surface.

6 Apr 2012
11:00
Christos N. LikosElectrostatics and soft matter: from star-branched polyelectrolytes to patchy colloids
Kolarjeva predavalnica (skupni seminar z Odsekom za elektronsko keramiko K5):
In this talk, we will first review the Physics of star-branched polyelectrolytes (SBP) demonstrating that their conformations and interactions are dictated by electrostatics in a rather subtle fashion, whereas the mutual repulsions of the charged monomers play the decisive role in stretching the arms, the subsequent effective interactions between SBPs is dominated by the entropic repulsion between the counterions adsorbed in the interior of the SBPs [1]. We will present the extensions of the theory to both spherical polyelectrolyte brushes [1] and to rigid chains [2,3], proceeding to the investigations on adsorption of PE-stars onto oppositely charged flat surfaces [4] or colloidal spheres [5]. We argue that the complexes that form can be viewed as a novel type of patchy colloids [6], and we present an efficient way of coarse-graining their effective interactions to a manageable yet realistic mesoscopic model [7]. Finally, we discuss ongoing work on soft patchy colloids, emerging out of end-functionalized starbust block copolymers [8]. [1] A. Jusufi and C. N. Likos, Rev. Mod. Phys. 81, 1753 (2009). [2] K. Kegler, M. Konieczny, G. Dominguez-Espinosa, C. Gutsche, M. Salomo, F. Kremer, and C. N. Likos, Phys. Rev. Lett. 100, 118302 (2008). [3] A. Wynveen and C. N. Likos, Soft Matter 6, 163 (2010). [4] M. Konieczny and C. N. Likos, Soft Matter 3, 1130 (2007). [5] C. N. Likos, R. Blaak, and A. Wynveen, J. Phys. Condens. Matter 20, 494221 (2008). [6] E. Bianchi, R. Blaak, and C. N. Likos, Phys. Chem. Chem. Phys. 13, 6397 (2011). [7] E. Bianchi, G. Kahl, and C. N. Likos, submitted (2011). [8] B. Capone, C. N. Likos, and F. LoVerso, in preparation. more...

20 Mar 2012
15:00
Adriano AmaricciNon-equilibrium stationary state formation in driven Hubbard model
Physics seminar room (106). Abstract:

I shall present a recent work concerning the non-equilibrium dynamics of a strongly correlated electrons systems in a static electric field, with the aim of identifying the conditions to reach a non-equilibrium stationary state (NSS).
I show that, for a generic electric field, the convergence to a stationary state requires the coupling to a thermostating bath, absorbing the work done by the external force. By following the real-time dynamics of the system, I also show that coupling to bath provides an essentially sufficient condition, i.e. NSS can be reached for almost any value of dissipation. I characterize the properties of the NSS in terms of some physical observables, pointing out the existence of an analogue of the Pomeranchuk effect. Finally, I map out a phase diagram of the system and I identify a dissipation regime for which steady current is largest for a given field.

14 Mar 2012
15:00
Hantao LuPhotoinduced phase transition in one-dimensional extended Hubbard model
Tea room F1. Abstract:

We illustrate one interesting example of photoinduced phase transitions in low-dimensional strongly correlated systems. By using time-dependent Lanczos method, the nonequilibrium process of the half-filled one-dimensional Hubbard model, driven by external laser pulse, is investigated. Charge carriers, namely, holons and doublons, can be generated during the pump. Starting from the spin-density-wave (SDW) side, we find that when the system is close to the boundary, by tuning the laser frequency and strength, a sustainable charge order enhancement, which is absence in the Mott insulating phase, can be spotted even after the pulse turned off. This phenomenon has its root in the spectral properties. From the spectrum analysis on small size systems, it can be shown that with increasing the nearest-neighbour interactions, accordingly, charge-order favorite eigenstates move from high energy regime to less dense low energy part, which leaves themselves prone to be picked up by laser pulse. The conditions of the emergence of charge density order and possible experimental realizations are discussed.

13 Mar 2012
15:00
Wataru KoshibaeReal time simulation of photo-excited state in junction of double-exchange systems: Theoretical design of high-efficiency solar-cell
Physics seminar room (106). Abstract:

The photo-induced insulator-to-metal (I-M) transition is studied by the numerical simulation of real-time quantum dynamics of a double-exchange model. We find a characteristic multiplication of particle-hole (p-h) pairs by a p-h pair of high energy during the I-M transition. To examine the conversion from the p-h pairs into electric energy, we perform the numerical study on the junction systems combined by the double exchange models. The numerical results reveal:
(i) the threshold behavior with respect to the intensity and energy of light,
(ii) p-h pairs are well separated and pair annihilation is suppressed,
(iii) enhancement of collected carrier by meta-stability of I-M transition.
The energy window of the p-h pair multiplication is also important for efficiency of the energy conversion. In the light of the theory, we propose a path to high-efficiency solar-cell by interacting electrons.

6 Mar 2012
15:00
Luca TubianaGeometrical and topological entanglement in packaged viral DNA
Physics seminar room (106). Abstract:

The packing of DNA inside bacteriophages arguably yields the simplest example of genome organisation in nature and understanding its physics is of primary importance for both the biophysical implications and the possible applicative ramifications [1-3].

Cryo-em studies on bacteriophages epsilon-15 [4] and phi-29 [5] showed that DNA is neatly ordered in concentric shells close to the capsid wall, while an increasing level of disorder was measured when moving away from the capsid internal surface. On the other hand the detected spectrum of knots formed by DNA that is circularised inside the P4 viral capsid showed that DNA tends to be knotted with high probability, with a knot spectrum characterized by complex knots and biased towards torus knots and against achiral ones [6,7].

We show, using advanced stochastic simulation techniques, that both the shell ordering and the knot spectrum can be reproduced quantitatively if one accounts for the preference of contacting DNA strands to juxtapose at a small twist angle, as in cholesteric liquid crystals. Furthermore the DNA knots we observe are strongly delocalized and, intriguingly, this is shown not to interfere with genome ejection out of the phage [8]. Starting by this observation, we investigate the interplay of geometrical and topological entanglement in knotted DNA rings confined inside a spherical cavity, using stringent and robust algorithms for locating knots [9]. We show that the complex interplay between the length of the knotted portion of DNA, the contour length of the DNA ring, and the radius of the enclosing sphere can be encompassed by a simple scaling argument based on deflection theory [10].

References

[1] Earnshaw WC, Harrison SC (1977) DNA arrangement in isometric phage heads. Nature 268:598-602.
[2] Gelbart WM, Knobler CM (2009) Virology. pressurized viruses. Science 323:1682-1683.
[3] Siber A, Bozic AL and Podgornik R (2011) Phys Chem Chem Phys. DOI:10.1039/c1cp22756d
[4] Jiang W, Chang J, Jakana J, Weigele P, King J and Chiu W (2006) Nature 439: 612-616
[5] Comolli LR, Spakowitz AJ , Siegerist CE, Jardine PJ, Grimes S, Anderson DL, Bustamante C, Downing KH (2008) Virology 371:267-277
[6] Arsuaga J, Vazquez M, Trigueros S, Sumners D, Roca J (2002) Proc Natl Acad Sci U S A 99:5373-5377.
[7] Arsuaga, J et al. (2005) Proc Natl Acad Sci U S A 102:9165-9169.
[8] Marenduzzo D, Orlandini E, Stasiak A, Sumners DW, Tubiana L, Micheletti C (2009) Proc Natl Acad Sci U S A 106:22269-22274.
[9] Tubiana L, Orlandini E, Micheletti C (2011) Prog Theor Phys supp 191:192-204
[10] Tubiana L, Orlandini E, Micheletti C (2011) PRL 107:188302-188305

17 Feb 2012
13:00
Professor Stefan ThurnerEntropy for complex systems - exploring the world beyond Shannon
Tea room F1. Abstract: In information theory the so-called 4 Shannon-Khinchin (SK) axioms uniquely determine Boltzmann Gibbs entropy as the one-and-only possible entropy. Physics is different from information in the sense that physical systems can be non-ergodic. To describe strongly interacting statistical non-ergodic systems -i.e. complex systems- with a thermodynamical framework, it becomes necessary to introduce generalized entropies. A series of such entropies have been proposed in the past. Until now the understanding of the fundamental origin of these entropies and its deeper relations to complex systems is unclear. Non-ergodicity explicitly violates the fourth SK axiom. We show that violating this axiom and keeping the other three axioms intact, determines an explicit form of a generalized entropy, $Ssim sum_i Gamma (d+1,1-clog p_i)$, where $c$ and $d$ are scaling exponents, uniquely describing a statistical system; Gamma is the incomplete Gamma function. All recently proposed entropies appear to be special cases. We next prove that each (!) statistical system is uniquely characterized by the pair of the two scaling exponents (c,d), which define equivalence classes for all (!) interacting and non-interacting systems, i.e. no other possibilities for entropies exist. The corresponding distribution functions are special forms of Lambert-$W$ exponentials containing -- as special cases Boltzmann, stretched exponential and Tsallis distributions (power-laws) -- all abundant in nature. This is an ab initio justification of the necessity of generalized entropies. We next show how the phase space volume of a system is related to its (generalized) entropy. We illustrate this with physical examples of spin systems on constant-connectency networks and accelerating random walks.

27 Jan 2012
12:00
John H. JeffersonQuantum Dot Spin Cellular Automata for Realizing a Quantum Process
Tea room F1. Abstract:

The singlet and triplet states of a two-electron quantum dot can be used as a qubit, whose spin can be measured by a single charge detection. I will show how to implement single and two qubit quantum gates by coupling such dots together to form quantum cellular automata, driven by the coherent dynamics of the electrons. Such automata therefore provide a rapid, deterministic and stable way of performing universal quantum computation.

 
 
 
 
 
 
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